Audit logging will show who the message is from and who it is to.
See the man pages for omshowaud and omconfaud,
If the messages are originating on your server and going off-site, your sendmail log will also have some valuable information in it (/var/log/maillog on RHEL or CentOS).
Service Router Rules will allow you to block the messages based on Subject.
Check out the Administration Guide at http://www.scalix.com/documents/Scalix_Administration_Guide_11.3.r3.pdf
There is a section entitled "Setting Message Delivery Rules on the Router"
You might also want to check out the Setup and Configuration Guide at http://www.scalix.com/documents/scalix_setup_and_configuration_guide_113.pdf; there is a section on Spam Protection.